I paid for and downloaded the program, but it has disappeared from my computer.
Are you using Norton? If so it removed it from your computer. Open Norton Internet Security, go to File Insight and tell Norton to change the "trust status" of the file to "Trust Now."
Also, 'reputation' in the Norton notice just means the file is too new to have a positive reputation by people who have installed it. Contact Norton and tell them you installed it, and it's fine. You'll be helping the 'reputation' of the file by doing that. Please see this:
Note it states:
The reputation-based system uses "the wisdom of crowds" (Symantec’s tens of millions of end users)
It also states:
If you believe a file has been mistakenly detected by WS.Reputation.1, you can submit a dispute at https://submit.symantec.com/dispute/